1. Field of the Invention
The field of invention relates to the storage of natural gas. More specifically, the field relates to a system and a method for storing and releasing natural gas during different periods of demand.
2. Description of the Related Art
Natural gas is the portable and preferred fuel of choice around the world. Natural gas burns more completely than other traditional fuels, including petroleum and coal; therefore, the combustion of natural gas is comparatively less harmful to the environment. Natural gas and similar products, including LNG, propane and other compressed-gas fuels are much more efficient in engine and turbine combustion systems.
Electricity generation with gas turbines is well understood and relatively inexpensive. Recirculation of hot, compressed exhaust gases can lead to thermal conversion efficiencies of well over 90%. Turbine co-generation facilities not only produce electricity and carbon dioxide, which is useful for enhanced oil recovery and other chemical processes, but also hot water and steam. Commercial and private consumers can both use these products.
With the broad availability of natural gas as a fuel, electricity generation using gas turbine technology is increasing. The locations of natural gas resources are not restricted to only a few geographical areas. Advances in fracturing deep shale reserves in Europe, Japan and the Americas show that natural gas is and will continue to be a primary fuel for industrialized societies for years to come. Electrical co-generation is the best way to provide the maximum benefits of the hydrocarbon fuel for consumer markets
When producing electricity or natural gas for non-commercial users, a significant problem arises for natural gas transportation networks: diurnal demand. People, unlike manufacturing plants or facilities, tend not to be steady energy users throughout the day. People consume greater amounts of electricity during the day and into the early evening and much less at night and into the early morning. The higher rates of consumption form a “peak period of demand” and the tower rate of consumption creates a “non-peak period of demand”. This daily trend occurs throughout the year. During different seasons, however, the length of each period (longer or shorter periods of natural light requiring lesser or greater amounts of artificial light, respectively) and the amplitude of the period (for example, greater amounts demanded at higher and lower temperatures versus more moderate temperatures) can change the amplitude of the demand during the diurnal period. The location of the demand also has an impact upon what the diurnal demand is like. In cooler environments, overall daily electrical and natural gas demand is lower in the summer months and higher in winter months as consumers use heating equipment. In warmer environments, the daily demand trends are opposite as consumer use air conditioning units to stay cool.
Swinging electrical and natural gas consumption—not only in daily use but also in seasonal differences—results in variability across the natural gas transportation and production system. Natural gas production is nearly constant. The supply-demand gap between natural gas production and total consumption results in a “gas demand lag”. The lag, without intervention, manifests itself as system pressure increases and decreases (“swings”) across the natural gas transportation system.
Electrical generation facilities prefer constant, high-pressure natural gas as a feedstock. Pressure swings in natural gas feed can damage the electrical generation equipment, especially rotational equipment, including gas turbines, due to sudden inappropriate feed-to-fuel ratios that cause equipment slowdowns while under load.
Past solutions to mitigate pressure swings include in-line compressors in the transportation systems. CNG (compressed natural gas) booster compressors that operate during peak demand periods attempt to maintain transportation system pressure. The loss of natural gas feed pressure can result in both downtime for electrical generators and dissatisfied public customers.
Using compression equipment increases operating expenses of the transmission system because the compression equipment operates at CNG transportation system pressure. The compression equipment also must be operable to tolerate the shift in daily operating temperatures. In-line compressors are expensive to maintain because they do not steadily operate: they start when system pressure is at a low threshold value and stop when system pressure is at a high threshold value.
Compressors, despite best maintenance practices, do inadvertently break down. Rotational equipment breakdowns sometimes are catastrophic, requiring weeks of downtime while delivering and tuning new units. The sudden loss of natural gas feed pressure from a malfunction can result in immediate downtime for downstream electrical generators and long-term dissatisfied public consumers.